1. The Field Of The Invention
The present invention broadly relates to boat propellers and, more particularly, to improved screw propellers adapted to be driven by a prime mover at high speeds and used in the propulsion of small power boats. The invention is further concerned with boats propelled by a propulsion unit including the propeller of the invention, and with outboard motors which are provided with the propeller of the invention as a component thereof. The invention also provides a method of operating a power boat having a propulsion unit including the propeller of the invention.
2. The Prior Art
Screw propellers have been used for many years in propulsion systems for various types of watercraft. Usually a propeller having a predetermined diameter and pitch is mounted adjacent the stern of the watercraft at or below the water line, and the watercraft is provided with a prime mover adapted to drive the propeller at the required number of revolutions per minute to generate the thrust needed to propel the watercraft at the desired speed. In order to generate a required amount of thrust from a given horsepower one may increase or decrease the diameter of the propeller, the pitch of the propeller blades, and/or the operating speed of the propeller in revolutions per minute. The amount of thrust that is developed in a given instance may be increased by increasing the diameter, operating speed and/or blade pitch of the propeller, and similarly, the amount of thrust may be decreased by decreasing the diameter, operating speed, and/or blade pitch. Additionally, salt water is heavier than fresh water, and thus a screw propeller develops more thrust in salt water than in fresh water when operated under a given set of conditions.
It is apparent from the foregoing that the optimum size, construction and design of screw propellers will differ from one type of watercraft to another. For instance, the massive propellers employed in propulsion systems for ocean going vessels and other commercial watercraft of medium to large size are of markedly larger diameter and have heavy blades with much thicker cross sectional dimensions than the propellers used for small commercial and recreational boats. The thick blades are very strong and under normal operating conditions, they are not subject to the mechanical damage, distortion, deflection and/or pitch change which is characteristic of the propellers described hereinafter for use with small commercial and recreational boats. Also, since ocean going and other relatively large commercial vessels are operated in deep water, the propeller diameter may be increased with little danger of the rotating blades striking submerged objects and being damaged thereby. Propellers of large diameters may be employed, and relatively low operating speeds in terms of revolutions per minute may be used to provide the necessary amount of thrust to propel the vessel. Operating the propeller at slower speeds tends to reduce turbulence and increase efficiency. Thus, as a general rule the propellers of large commercial vessels are operated at markedly slower speeds than those of small commercial and recreational boats. The initial cost of propellers for large commercial vessels usually is not an important factor in the overall cost of the vessel, and high cost metals and metal alloys may be used as materials of construction.
Screw propellers used in the propulsion of small commercial and recreational boats differ markedly from the above described propellers for large vessels and must meet a number of special requirements from the standpoint of size, construction and design, and also from the standpoint of economics. Small boat propellers have diameters of about twenty inches or less and thin blades with a maximum thickness of about one inch or less, and therefore they are much lighter in weight and the blades tend to have less strength. Small boat propellers are operated at high speeds in order to provide sufficient thrust, such as up to about 9,000 revolutions per minute, and especially when used in a propulsion system for a recreational high speed boat. When a boat propeller is operating at high speed, there is a tendency for the thin, blades to deflect, distort and/or lose their pitch due to the increased stress. This in turn decreases the amount of thrust which would otherwise be available for a given speed of operation. As a result, it is advisable to construct boat propellers that are about thirteen inches and larger in diameter from stainless steel in order to have sufficient strength in the blades. Smaller diameter boat propellers are usually constructed from aluminum, bronze or plastics. However, even these small diameter propellers are often subject to loss of pitch at operating speeds approaching 6,000 to 9,000 revolutions per minute. There is also a problem with respect to damage of the blades when objects are struck during high speed operation due to the relatively low strength of the thin blade sections. A stainless steel propeller of comparable size is a definite added advantage in this respect, but stainless steel small boat propellers are so expensive that they are not normally used except when necessary. Prior to the present invention most small boat owners used the less costly propellers prepared from aluminum though they were not generally satisfactory.